(1) Field of the Invention
The present invention relates to an adhesive resin composition and a laminate comprising this resin composition as an adhesive layer. More particularly, the present invention relates to an adhesive resin composition giving an excellent adhesiveness between a metal and a thermoplastic resin, and a metal/resin sheath laminate to be used for a laminate sheath cable, which is formed by using this resin composition. Furthermore, the present invention relates to an adhesive resin composition, of which the adhesive force is not reduced even under contact with high-temperature water, and a laminate having an excellent gas-barrier property and being capable of resisting a retort treatment, which is formed by using this adhesive resin composition.
(2) Description of the Related Art
An adhesive resin composition comprising three components, that is, a styrene resin, an ethylene/vinyl acetate copolymer resin and a polyethylene resin graft-modified with an unsaturated carboxylic acid or a derivative thereof has been publicly known. This adhesive resin composition has an excellent adhesiveness between a metal and a thermoplastic resin and also between a polyester resin and an ethylene/vinyl acetate copolymer, which are used for a packaging material or the like to be subjected a retort treatment.
This adhesive resin composition is used for a laminate sheath cable having a resin sheath arranged on the outer side of a barrier material for a cable and a cable core, and since this laminate sheath cable is excellent in such characteristics as mechanical properties, corrosion resistance and moisture resistance, this cable is widely used at the present. In this laminate sheath cable, the above-mentioned adhesive resin composition is used for bonding a metal tape composed of aluminum, copper or the like, to be used as the barrier layer, to a sheath resin composed mainly of low-density polyethylene.
The laminate metal tape is generally prepared by extrusion lamination of a metal and a sheath resin or bonding a metal layer to a fusion-bonding resin layer film and a sheath resin layer film. In case of conventional fusion-bonding resins, streaks are often formed on the laminated fusion-bonding resin layer.
These streaks are formed because of local changes of the thickness of the fusion-bonding resin layer and the presence of these streaks degrades the adhesion between the metal and sheath resin and reduces the appearance characteristics of the product. Moreover, cracks are formed in a thin portion of the fusion-bonding resin layer and even the performances of the product are reduced.
In (Japanese Unexamined Patent Publication No. 61-296044, we previously proposed a thermoplastic resin composition capable of preventing formation of streaks in the fusion-bonding resin layer, which comprises 97 to 45 parts by weight of an ethylene/vinyl acetate copolymer, 30 to 1 parts by weight of a styrene polymer resin, 15 to 1 parts by weight of polyethylene graft-modified with an unsaturated carboxylic acid or a derivative thereof and 30 to 1 parts by weight of a monovinyl aromatic hydrocarbon/olefin block copolymer. When a laminate metal tape is prepared by using this thermoplastic resin composition as the fusion-bonding resin, streaks are not formed in the fusion bonding resin layer, but the adhesiveness between the metal and a resin sheath layer is not completely satisfactory.
Accordingly, development of an adhesive resin composition which can prevent formation of streaks in the fusion-bonding (adhesive) resin layer and can give an excellent adhesiveness between a metal and a resin sheath layer is eagerly desired.
Although the above-mentioned adhesive resin composition is used for the production of a laminate of a polyester resin and a gas-barrier resin to be used for a packaging vessel or the like and shows a good adhesiveness, if a heat treatment such as a high-temperature filling treatment or a retort treatment is carried out at the production of this laminate or at the time of eating or drinking a content in the packaging vessel, the adhesive force is sometimes reduced by heating, resulting in peeling of the layer and reduction of the gas permeation resistance.
As the adhesive resin composition capable of retaining a high adhesive force between layers of a laminate even after a high-temperature treatment such as a high-temperature filling treatment or a retort treatment, we previously proposed in Japanese Unexamined Patent Publication No. 64-45445 an adhesive resin composition comprising (a) 95 to 50% by weight of an ethylene/.alpha.-olefin copolymer having a melt flow rate of 0.1 to 50 g/10 min, a density of 0.850 to 0.900 g/cm.sup.3, an ethylene content of 75 to 95 mole % and an X-ray crystallinity lower than 30%, (b) 5 to 50% by weight of an ethylene/vinyl acetate copolymer having a melt flow rate of 0.1 to 50 g/10 min and a vinyl acetate content of 5 to 40% by weight, and (c) 1.0 to 30% by weight, based on the sum of components (a) and (b), of partially or wholly graft-modified polyethylene having a grafting amount of an unsaturated carboxylic acid or a derivative thereof of 0.05 to 15% by weight, a melt flow rate of 0.1 to 50 g/10 min, a density of 0.900 to 0.980 g/cm.sup.3 and an X-ray crystallinity of at least 30%, wherein the grafting ratio of the composition as a whole is 0.01 to 3% by weight, the melt flow rate of the composition as a whole is 0.1 to 50 g/10 min and the crystallinity of the composition as a whole is lower than 35%. In this patent publication, we also proposed a laminate comprising a polyester or polycarbonate layer, an intermediate layer composed of this adhesive resin composition and a saponified olefin/vinyl acetate copolymer layer.
The above-mentioned adhesive resin composition has an excellent adhesiveness at normal temperature after a high-temperature treatment, but interlaminar peeling is sometimes caused in the above-mentioned laminate during a high-temperature filling treatment or a retort treatment.
Accordingly, development of an adhesive resin composition capable of completely preventing occurrence of interlaminar peeling in a laminate even under a high-temperature filling treatment or a retort treatment is desired.